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FPS to Shutter Speed (180° Rule)の包括的な教育ガイドを準備中です。ステップバイステップの解説、数式、実例、専門家のヒントをお届けしますので、もうしばらくお待ちください。
The FPS to Shutter Speed Calculator implements the 180-degree shutter rule — the fundamental motion blur principle used in professional film and video production. The 180-degree rule states that the camera's shutter speed (or more precisely, the shutter angle) should be set to double the frame rate to produce natural-looking motion blur that mimics the human eye's perception of movement. At 24 fps, the correct shutter speed is 1/48 s (approximately 1/50 s in practice); at 30 fps, it's 1/60 s; at 60 fps, it's 1/120 s. The rule originates from film camera mechanics: a rotating disc shutter with a 180-degree opening exposes the film frame for exactly half the time between frames, naturally producing 180-degree equivalent motion blur. This 50% exposure ratio creates motion blur that the human visual system interprets as smooth, natural movement — matching what our eyes perceive when watching real motion. Shutter angles other than 180 degrees are used for creative effect: a smaller angle (e.g., 90 degrees = 1/4× frame period) produces sharper, more stroboscopic motion reminiscent of Saving Private Ryan's combat sequences; a larger angle (e.g., 270 degrees) produces heavier blur for dreamlike or impressionistic sequences. The calculator converts between frame rate, shutter angle, and shutter speed, helping videographers choose the correct exposure setting for any frame rate and creative intent. Understanding this rule is foundational to cinematography and professional video production.
Standard Shutter Speed = 1 / (fps × 2) [180° rule] General Shutter Speed = 1 / (fps × (360 / Shutter Angle)) Shutter Angle = 360 × fps × Shutter Speed For 180° rule: Exposure Time = 1 / (2 × fps)
- 1Step 1: Determine your frame rate (fps). Common choices: 24/25 fps for cinematic, 30 fps for broadcast, 60/120 fps for slow motion.
- 2Step 2: Apply the 180° rule: shutter speed = 1/(fps × 2). At 24 fps: 1/48 s; at 30 fps: 1/60 s; at 60 fps: 1/120 s.
- 3Step 3: Set the camera to the nearest available shutter speed. Most cameras offer 1/50 s as the closest to 1/48 s (acceptable for 24/25 fps).
- 4Step 4: Adjust exposure using ISO and ND filters — never adjust shutter speed to control exposure when following the 180° rule.
- 5Step 5: For creative shutter angles, use the formula: shutter speed = shutter angle / (360 × fps). At 90° and 24 fps: 1/(360/90 × 24) = 1/96 s.
- 6Step 6: When shooting slow motion at high fps (120, 240 fps) for normal-speed playback, apply the 180° rule at the capture fps, not the playback fps.
1/(24 × 2) = 1/48 s. Most cameras offer 1/50 s as the nearest standard. The slight difference is imperceptible. This is the 'film look' shutter speed used for 24 fps cinema production.
1/(30 × 2) = 1/60 s. Exactly achievable on all cameras. Standard for US television production at 29.97/30 fps.
1/(120 × 2) = 1/240 s. When slowed to 24 fps in post (5× slowdown), motion blur per frame looks natural. Requires bright light or very high ISO due to fast shutter.
45/360 × 1/24 = 1/192 s. Director Spielberg and DP Janusz Kaminski used a 45° shutter angle for the opening D-Day sequence to create a hyper-real, stroboscopic motion effect.
Cinematographers setting shutter speed for narrative film and commercial video production.
Wedding and event videographers choosing correct settings for natural motion rendition.
Sports videographers selecting frame rate and shutter speed for slow-motion highlight reels.
Documentary filmmakers matching shutter angle to venue lighting frequency to avoid flicker.
HFR (High Frame Rate) cinema
Peter Jackson's 'The Hobbit' (2012) was shot at 48 fps (HFR) — some screenings also projected at 48 fps for unprecedented clarity. At 48 fps with 180° shutter, the speed is 1/96 s. HFR cinema remains controversial: audiences often find the hyper-realistic motion unsettling ('soap opera effect') because the brain expects cinema's 24 fps temporal rhythm.
Flicker with artificial light
At certain shutter speeds, artificial lighting (fluorescent, LED, sodium vapor) can flicker because their frequency is a multiple of local mains electricity (60 Hz in North America, 50 Hz in Europe). Safe shutter speeds are multiples of 1/60 s (60 Hz countries) or 1/50 s (50 Hz countries). At 24 fps with US power: 1/48 s is slightly off from 1/60; some flickering may occur. Setting 1/60 s (slightly violating 180° rule) eliminates flicker.
When input values approach zero or become negative, the Fps Shutter Speed
When input values approach zero or become negative, the Fps Shutter Speed calculation may produce undefined or misleading results. Always validate that inputs fall within the model's valid range before interpreting outputs. Extreme values should be flagged for manual review.
| Frame Rate (fps) | 45° (sharp) | 90° | 180° (standard) | 270° |
|---|---|---|---|---|
| 24 | 1/192 s | 1/96 s | 1/48 s | 1/32 s |
| 25 | 1/200 s | 1/100 s | 1/50 s | 1/33 s |
| 30 | 1/240 s | 1/120 s | 1/60 s | 1/40 s |
| 48 | 1/384 s | 1/192 s | 1/96 s | 1/64 s |
| 60 | 1/480 s | 1/240 s | 1/120 s | 1/80 s |
| 120 | 1/960 s | 1/480 s | 1/240 s | 1/160 s |
| 240 | 1/1920 s | 1/960 s | 1/480 s | 1/320 s |
Why is the 180-degree rule considered standard in film?
The 180-degree rule produces motion blur that is perceptually matched to what the human eye perceives when watching real motion. At this shutter angle, moving objects blur across approximately 50% of their travel distance per frame, creating the smooth, continuous sense of motion that cinema audiences have been conditioned to expect from 100+ years of film production. Different motion blur feels 'wrong' even to untrained viewers.
Can I break the 180-degree rule for creative effect?
Absolutely — intentional deviation from 180° is a legitimate creative tool. Lower shutter angles (45°–90°) create sharper, more stroboscopic motion used for action sequences, war films, and sports (Ridley Scott, Janusz Kaminski, Roger Deakins have all used tight shutter angles for effect). Higher shutter angles (270°–350°) create dreamy, impressionistic blur. The rule is a baseline for natural-looking motion, not a rigid law.
What is the difference between shutter angle and shutter speed?
Shutter angle is a frame-rate-independent way to express exposure duration, originating from mechanical film cameras with rotating disc shutters. A 180° shutter angle always produces an exposure of half the frame period, regardless of frame rate. Shutter speed (e.g., 1/50 s) is an absolute exposure duration. For a given frame rate, they are equivalent: Shutter Speed = Shutter Angle / (360 × fps). Modern digital cinema cameras express shutter in both systems.
How do I control exposure when the shutter is locked at the 180° rule setting?
When following the 180° rule, adjust exposure using ISO, aperture, and ND filters — never by changing shutter speed. In bright conditions, use ND filters (ND8 or ND64) to reduce incoming light. In low light, increase ISO and open aperture. A variable ND filter is invaluable for video production, allowing continuous exposure adjustment while maintaining the locked shutter speed.
What shutter speed should I use for 25 fps (European PAL standard)?
At 25 fps (standard for European and UK broadcast), the 180° rule gives 1/50 s — exactly available on all cameras and a common European frequency related to 50 Hz mains electricity (used to avoid fluorescent light flicker). This is one reason 25 fps became the PAL standard: 1/50 s shutter speed is synchronized with European electrical frequency, minimizing fluorescent light flickering in interiors.
Does the 180-degree rule apply to photography?
In still photography, the 180° rule does not apply because individual frames are intended as frozen moments rather than as part of a motion sequence. Still photographers choose shutter speed based on desired motion blur effect (freezing action vs. creative blur) and camera shake limits (handholding rule: minimum 1/focal_length seconds). The 180° rule is specific to video and cinema workflows.
How does frame rate affect slow-motion playback?
Slow-motion effect = capture fps / playback fps. Shooting at 120 fps played back at 24 fps gives 5× slow motion. The 180° rule applies at the capture frame rate: at 120 fps, use 1/240 s shutter. When reviewing slow-motion footage in real time on set, the 5× slower playback will show 5× more motion blur per second, but each individual frame maintains the correct motion blur for natural-looking slow motion.
プロのヒント
Many modern mirrorless cameras (Sony, Canon, Fujifilm) have an Auto ISO with minimum shutter speed lock — set this to the appropriate 180° rule speed for your frame rate so the camera adjusts ISO while keeping shutter speed fixed for cinematic motion blur.
ご存知でしたか？
The term 'cinematic look' is largely a result of 24 fps + 1/48 s (180° rule) motion blur. Video shot at 60 fps with 1/60 s (sharp motion) is perceptually associated with news, sports, and soap operas — genres considered 'uncinematic.' The motion blur amount, not the frame rate alone, defines the look.
参考文献
- ›SMPTE ST 2067 – Interoperable Master Format (IMF) Frame Rate Specifications
- ›ARRI Cinematography Guide: Shutter Angle and Motion Blur
- ›Roger Deakins CBE: Cinematography Q&A (Shutter Angles)
- ›ASC (American Society of Cinematographers): Digital Cinematography Handbook
- ›DPReview: Video Shutter Speed and Motion Blur Explained